Syria was hit by a third car bombing of the weekend as political security offices in the northern city of Aleppo were targeted, according to human rights groups.

State media, which have said that such attacks aimed to sabotage efforts to find a political solution to Syria's crisis, said it exploded near residential buildings and a post office.

Activists in Aleppo, the target of car bombings on February 10 that killed 28 people, told AFP in Beirut on Skype that the blast rocked the city early in the afternoon.

On Saturday, twin car bombings killed 27 people and wounded 140 others in the heart of Syria's capital, the interior ministry said, blaming "terrorists" for the attacks near police and air force headquarters.

"Yesterday's explosions were carried out by terrorists supported by foreign powers which finance and arm them," charged Al-Baath newspaper, mouthpiece of President Bashar al-Assad's ruling party of the same name.

"The two attacks ... aim to disrupt Annan's mission and to foil international efforts to find a political solution to the crisis," it said, referring to UN-Arab League peace envoy Kofi Annan.

A rally and prayers were being held on Sunday at the site of the biggest explosion, in the Al-Qasaa district that is home to many members of Syria's Christian minority.

State television, which said victims were being buried Sunday, has repeatedly broadcast how the Al-Qasaa blast had totally gutted the facade of a multi-storey building, wrecked family homes and left behind blood-splattered pavements.

Opposition activists accused the regime, as in past lethal bombings in the capital and the northern city of Aleppo, of having stage-managed the attacks.

Ath-Thawra, another official daily, pointed the finger at Qatar and Saudi Arabia which have called for rebels fighting the Assad regime to be armed.

"The terrorism of Hamad and Saud is not a first. We know their bloodstained hatred, born of jealousy ... We have heard their call, and their incitement," it said, referring to the Saudi and Qatari ruling families.

Also in the press on Sunday, a daily close to the regime warned of an even tougher response to anti-regime demonstrations.

In the wake of the Damascus bombings, "this is no longer the time for philosophising, for struggle, for Facebook ... and especially not for protests which only go to serve the enemies of Syria," Al-Watan said.

Technical experts from the United Nations and the Organisation of Islamic Cooperation, meanwhile, were to take part in a mission to assess the humanitarian impact of the crackdown on anti-regime protests since March 2011.

UN humanitarian chief Valerie Amos, who held talks in Damascus this month, said they would at the weekend join the assessment mission to Daraa, Homs, Hama, Tartus, Latakia, Aleppo, Deir Ezzor and rural zones around Damascus.

The United Nations estimates more than 30,000 Syrians have fled to neighbouring states and another 200,000 have been displaced within the country by the past 12 months of deadly violence.

In the Aleppo region, the town of Atareb was shelled for the 33rd straight day, said an activist on the ground, Mohammed al-Halabi, contacted by AFP in the Lebanese capital.

The Syrian Observatory said four soldiers were killed in clashes with rebels in Idlib province, while security forces killed a civilian in Daraa.

It also reported that security forces beat up and detained opposition figure Mohammed Sayyed Rassas, a National Coordination Committee for Democratic Change leader, and several youths at a Damascus protest.

On Saturday, two "terrorists" were killed as a booby-trapped car they were driving blew up in a Palestinian refugee camp in a suburb of Damascus, Syria's state news agency SANA reported.

International envoy Annan on Friday warned of a regional "escalation" of the conflict which activists say has cost more than 9,100 lives and urged the UN Security Council to close ranks to put pressure on Assad.

The former UN chief, who met Assad in Damascus last weekend, has ordered a team of experts to Syria to discuss a possible ceasefire and international monitoring mission, his spokesman said.

Annan's team are to head to Damascus from New York and Geneva on Monday, his spokesman said.